Designers of beverage vendors, especially apparatuses for preparing coffee or tea by extraction, have long been of two problems for which a variety of solutions has already been offered.
The first problem is to establish a predetermined extraction period during which the extraction process takes place in the extraction chamber and during which preferably no fluid will seep through the filter into the reception chamber, followed by a quick transfer of the ready prepared extract from the extraction chamber through the filter into the reception chamber. During the extraction process the volume of water involved has to be constant to assure a manageable and reproducible extraction process. However, the moment the prepared extract has exactly the required taste and flavour, this extract has to be transferred through the filter into the reception room as quickly as possible to avoid any undesired change in taste or flavour due to a prolonged extraction process.
The second problem is maintaining the coffee or tea particles in motion within the water during the extraction process to obtain a quick, uniform homogeneous extraction. In other words, the second problem is how to achieve an agitation of the mixture of the hot water and the coffee or tea powder in the extraction chamber during the extraction period.
A type of apparatus in which a solution is offered for both problems is the apparatus comprising a piston movable in a cylindrical reception chamber. An apparatus of this type is for instance described in the Dutch patent application 68.06368, in the European patent specification EP 0 179 866 and in the British specification GB 2 178 645.
In this type of prior art apparatus the reception chamber has a cylindrical shape and comprises a piston connected through a piston rod to a crank shaft which can be driven by a motor. During the extraction period the piston is moved upwards so that air will be pressed from the reception chamber through the paper filter web of the filter unit into the extraction chamber, thereby agitating the mixture of hot water and coffee or tea powder. This agitation of the mixture has a beneficial influence on the quality of the prepared extract. Thereafter the piston is used for suctioning the prepared extract through the filter unit into the reception chamber for which purpose the piston will be moved downwards from its highest position, reached during the extraction period. As a result of the underpressure created thereby in the reception chamber, the extract will be suctioned from the extraction chamber through the filter unit into the reception chamber and can be discharged through a discharge channel opening into the reception chamber.
A first disadvantage of this prior art apparatus is its relatively complicated piston mechanism with a lot of rotatable parts, which is difficult to maintain and to clean. In particular, the fit of the piston in the cylindrical reception chamber has to be maintained within certain tolerance limits to guarantee that this prior art apparatus will function properly.
A further disadvantage is the invariability of the functioning of the apparatus. It is impossible or practically constant to have any influence on the length of the time period in which air is pressed through the mixture of water and coffee or tea powder, and the length of the suction period during which the prepared extract is suctioned through the filter unit into the reception chamber. Also, the pressure which is developed in both cases is constant or practically constant because of the inverse relation between pressure and time dictated by the use of the moving piston to create the pressure.
From the prior art, other proposals for solving the above-mentioned problems are known in which no moving piston is used but use is made of a pump. In the apparatuses described in U.S. Pat. Nos. 3,446,137 and 3,666,558 a pump is used for generating an overpressure in the extraction chamber at the end of the extraction period. This overpressure causes a quick transfer of the prepared extract from the extraction chamber through the filter into the reception chamber. This prior art type apparatus has the disadvantage that means have to be used for closing the entrances through which the hot water and the coffee or tea powder are brought in the extraction chamber in order to generate an overpressure in said extraction chamber. In addition in most cases the extraction chamber has to be movable upwards to provide space for moving parts of the filter unit to dispose the used coffee or tea powder after completion of the extraction process which may cause mechanical problems in case a pump has to be connected to the extraction chamber. Further-more, this prior art apparatus does not comprise means to agitate the mixture in the extraction chamber.
This last-mentioned disadvantage is eliminated in the apparatus described in U.S. Pat. No. 3,338,153 comprising a number of additional valves and further piping which enable the pump to be used during the extraction period for generating an overpressure in the reception chamber. This causes a steady air flow through the filter and through the mixture of water and coffee powder upwards resulting in the desired agitation of the mixture. The other above-mentioned disadvantages caused by connecting a pump to the extraction chamber are still present in this prior art apparatus. Furthermore, the extensive piping running to both the extraction chamber and to the reception chamber is considered another disadvantage of this apparatus.
A proposal to overcome the use of extensive piping and a large number of additional valves is described in WO 8.802.612. The apparatus described therein comprises one pump connected to the extraction chamber, used for suctioning air from said chamber during the extraction process such that an upward airstream through the extracting mixture is obtained and used for pressurizing the extraction chamber after the extraction period to press the extract through the filter. Although this prior art apparatus comprises means both to agitate the mixture during the extraction period and to quickly transfer the ready mixture at the end of the extraction process, if still possesses a lot of the above-mentioned disadvantages.
In the past, designers had already proposed to use a pump for generating an underpressure in the reception chamber to suction the extract through the filter paper web into the reception chamber. Examples of such apparatuses are described in GB 806.822, DE 2.043.803 and DE 3.019.347. In all these prior art apparatuses the extract is flowing through the pump. Therefore, the whole construction of the pump has to be such that contact between various parts of the pump and beverages is acceptable. This requirement restricts the choice of suitable pumps. Another problem arises from the fact that the extract is in fact not accumulated in the reception chamber but flows directly further through the discharge conduit. However, the flow velocity will be rather high, in some cases too high to deliver the extract directly into a cup. A further disadvantage of these prior art apparatuses is the lack of means for agitating the mixture in the extraction chamber.